The invention relates to a cylinder head gasket formed of a single metal base plate, which is installed between two members, such as a cylinder head and a cylinder block, of an internal combustion engine to seal therebetween.
When a joint surface between the cylinder head and the cylinder block (cylinder body) of the engine of the automobile is sealed, the cylinder head gasket is installed therebetween to seal combustion gas, cooling water and the like.
In view of demand for a light weight and low cost of the engine, a structure of the cylinder head gasket has been shifted to a simple type formed of a single metal base plate from a laminated type having a number of metal plates. Due to a single plate structure, only limited types of materials can be used.
In the laminated type metal plate gasket, a sealing member, such as a bead, grommet and shim, can be combined as necessity and provided in double for effective sealing. However, in the single plate gasket, a type and a number of sealing members are limited, so that a simplified sealing device has to be used. Moreover, an area available for the sealing device has been limited with reduction of an engine size.
As shown in FIG. 5, the cylinder head gasket is formed to have a shape of an engine member such as a cylinder block. The cylinder head gasket includes holes 2 for cylinder bores (hereinafter referred to simply as xe2x80x9ccylinder bore 2xe2x80x9d); fluid holes 3, 4 for circulating cooling water and engine oil (hereinafter referred to simply as xe2x80x9cfluid holes 3, 4xe2x80x9d); and bolt holes 5 for tightening bolts and the like. Also, sealing devices, such as beads 12, 13, with respect to the respective holes to be sealed are provided.
Although the same gasket is used to seal between the engine components, a different sealing property is required to seal the different holes. At the cylinder bores 2, it is required to seal the combustion gas with a high temperature and a high pressure, while a fluid with a relatively low temperature and a low pressure needs to be sealed at the fluid holes 3.
An arc shape bead 12 with a narrow width, which is formed of a material with elasticity for fitting to an irregular surface, is provided around the cylinder bore 2. Also, the cylinder bore 2 is surrounded by the bolt holes 5. Bolts are inserted into the bolt holes 5 to tighten the cylinder head and the cylinder block by applying a large pressure.
A bead 13 is provided around the fluid hole 3 to seal the same. Depending on a structure of the engine, the fluid hole 3 may be formed outside an area surrounded by the bolt holes 5. In this case, a pressing force of the tightening bolts is applied on only one side of the fluid hole 3, so that the tightening force becomes small. To solve the problem, a higher bead, a narrower bead, or a pointed bead has been used.
Further, to seal a small uneven surface like a tool mark or the like formed on a sealing surface of the engine member, such as the cylinder head and the cylinder block, a micro seal coating made of rubber, such as a NBR rubber and a fluoro rubber, is applied to entire surfaces on both sides of the gasket.
Since the micro seal coating generally has a hardness of 2H-4B in pencil hardness and a thickness of 10 xcexcm-30 xcexcm, the micro seal coating has a sufficient sealing effect with respect to a small uneven surface in the order of 5 xcexcm-15 xcexcm, such as a tool mark left on the sealing surface of the engine member. However, the micro seal coating can not effectively seal a slightly larger uneven surface like a V-notch and a step with a 20 xcexcm-50 xcexcm depth.
Especially, in the case that such a large scratch formed on the surface of the engine member crosses a sealing line formed by the bead or the like, a part of the scratch is not sufficiently sealed. Thus, the combustion gas, cooling water or oil may leak.
The leaking problem by the scratch, such as a V-notch, on the sealing surface of the engine member has been considered as a serious problem. This is because a relatively soft and fragile metal such as an aluminum alloy has been used for an engine member to reduce a weight of an engine. Also, even slight leak of the combustion gas, cooling water or oil needs to be prevented.
In view of the above problems, the present invention has been made. An object of the invention is to provide a cylinder head gasket formed of a single base plate, wherein the cylinder head gasket has a primary sealing portion having resistance to the combustion gas with a high temperature, and a secondary sealing portion for forming a seal line with an appropriate hardness to thereby obtain a good sealing ability and durability.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In order to attain the above objects, a cylinder head gasket of the present invention is structured. In the first aspect of the invention, a cylinder head gasket is formed of a single metal base plate for sealing between a cylinder head and a cylinder block of an engine. The cylinder head gasket has a primary sealing portion including a grommet or shim around a cylinder bore, and a secondary sealing portion outside the primary sealing portion. The secondary sealing portion is formed of a bead and a foamed coating layer coated on one side or both sides of the bead to cover at least a portion of the bead in a belt shape.
The metal base plate is made of such a material as annealed stainless steel, heat-treated stainless steel (spring steel) and soft steel. The grommet or the shim is made of the annealed stainless steel, the soft steel or copper.
While the bead disposed on the secondary sealing portion is generally formed in a full bead, such as a circular arc or a trapezoidal shape, other type, such as a half bead, may be used depending on a sealing ability required for the engine.
The foamed coating layer is formed by coating a foam synthetic resin, such as a silicone resin containing micro capsules, on a surface of the metal base plate by a screen-printing at a foaming ratio of 200%-300%, with a thickness of 100 xcexcm-200 xcexcm and a width of 1 mm-5 mm. While the foamed coating layer is applied away from the grommet or shim of the primary sealing portion with a small space therebetween, the foamed coating layer may be connected to the grommet or shim. When there is an enough space, the foamed coating layer may be disposed with an appropriate space therebetween.
According to the structure, the grommet or shim of the primary sealing portion can withstand a high temperature of the combustion gas. Moreover, the grommet or shim has hardness to prevent the gasket from creep relaxation. In the secondary sealing portion, a secondary sealing line is formed of the bead and the foamed coating layer to thereby improve the sealing effect.
Also, since the foamed coating layer of the secondary sealing portion has a thickness greater than that of a micro seal coating layer, a difference in height created by the primary sealing portion can be absorbed. Moreover, since the foamed coating layer on the gasket surface covers the bead to contact the engine members, and has a relatively soft property for fitting, a sufficient sealing effect is achieved with respect to a scratch, such as a V-notch, larger than a tool mark, formed on the sealing surface of the engine member, such as the cylinder head.
Also, since the primary sealing portion is formed of a narrow sealing device, such as the grommet or shim, it is possible to apply to a smaller area associated with a reduced size of an engine. Further, the primary sealing portion prevents the high temperature combustion gas from contacting the secondary sealing portion to thereby protect the secondary sealing portion.
As the second aspect of the invention, the cylinder head gasket has a fluid hole sealing portion provided around the fluid hole. The fluid sealing portion is formed of a bead and a foamed coating layer coated on one side or both sides of the bead to cover at least a part of the bead in a belt shape.
The bead in the fluid hole sealing portion is generally formed in a half bead, since water or oil is sealed and the pressure thereof is not high. However, depending on a sealing property required for the engine, other shape, such as a full bead, may be used.
In the above structure, the fluid hole sealing portion can be formed by the same process as that of the secondary sealing portion of the cylinder bore, thus an additional production process can be eliminated to thereby reduce a production cost.
In the third aspect of the invention, the cylinder head gasket has a foamed coating layer in the secondary sealing portion. The foamed coating layer has a thickness thicker than that of the foamed coating layer in the fluid hole sealing portion.
The thickness of the foamed coating layer can be easily changed by a double coating process. Therefore, it is easy to design various sealing properties for the cylinder bore and the fluid hole.
Also, the foamed coating layer in the fluid hole sealing portion may have a higher foaming ratio than that in the secondary sealing portion.
In the cylinder head gasket, a micro seal coating layer is applied to the whole surface of at least one side of the cylinder head gasket where the foamed coating layer is coated. The micro seal coating is made of such a material as NBR rubber and fluoro rubber, and is applied to the substantially whole surface of the gasket by the screen printing to form a thin film having a hardness of H-2B of pencil hardness and a thickness of 10 xcexcm-30 xcexcm.
The micro seal coating provides a sealing effect with respect to an uneven surface with a micro depth of 5 xcexcm-15 xcexcm, such as a tool mark left on the sealing surface of the engine members. Also, since the foamed coating can be fixed on the surface, a flow of the foamed coating and creep relaxation due to the flow can be prevented.
In the cylinder head gasket, the foamed coating layer is formed of a foamed silicone coating. The micro seal coating layer is formed of a fluorine or NBR rubber coating.
Although other materials can be used for the foamed coating layer and the micro seal coating layer, the above described combination is especially preferable in view of the sealing ability and production.